The brick fortifications were erected in the first half of the 14th century, and their construction continued for another 50 years. Firstly, the lower parts of the three main gates were constructed. The second stage of the fortifications lasted from 1348 to 1385 and at that time the gates and walls from the south, west and north were raised. Eastern section on the Słupia river was erected at the beginning of the 15th century.
In 1441, prince Bogusław allowed the extension of the water system used to defend the town. Probably at the same time there was a further modernization of the fortifications. Changed the upper parts of Mill and New gates and the so-called Witches Tower located in the east, and the gates of New and Holstein have been strengthened with foregates. In the east the Smiths Gate was erected, simplifying the entrance to the town by a levee built on a wet meadows. More serious works on early modern earth fortifications were made in the town only in 1624 in the face of the 30-year war. However, these works were not completed and were only realized by the Swedes after the seizure of the town in 1630.
Medieval fortifications, deprived of their original functions, began to fall into ruin. They were overbuilt by residential houses and economic buildings or stripped of brick. In 1834 the Powder Tower was dismantled, and in the 1860s the Holstein Gate and significant parts of the walls, in the north and west.
The ring of fortifications had the shape of an irregular trapezoid with a broader side based on the riverbed of the Słupia, flowing from the east. The area of the town was about 15 hectares, and the circumference of the walls was about 1600 meters. In the Middle Ages, the town was surrounded by a underwall street. The earlier wall was made of bricks in monk and flemish bond on the foundations of stone boulders. Those narrowing up walls did not have a arrowslits or battlement. Their original height is unknown. The highest preserved parts today are 6,5 meters high and 1 meter thick. The later wall in the eastern side of the town reached 8 meters high and probably due to the wetland area above Słupia river had a different construction. In the lower parts of it, are visible characteristic construction bows supported on a stone foundation. The exterior part of this wall was erected vertically, and on the interior side there is a fairly wide section, which once housed the guard porch.
The wall was strengthen with two types of towers. In the wall of the earlier phase there were rectangular half towers, open from the side of the town, spaced at intervals of 15 to 30 meters. These half towers had an average size of 6,5×2,8 meters; thickness of the wall from 1 to 1,6 meters, and they were extended out before the wall about 60 cm. On the outer façade of the towers there were decorative, vertical niches in the number 4 or 5. In the interiors of the towers floors are preserved with usually two arrowslits above them. In some of the northern towers in the lower levels, two fire embrasures were added later. In addition to the above mentioned half towers, there are also half towers of a much smaller size and only slightly extend beyond the outer line of the wall, devoid of arrowslits and storeys. There are only two towers present in the eastern wall dating back to the 15th century. One set on circural plan with the significant wall thickness of about 1,6 meters and the second, so-called Witches Tower. Built probably around 1410-1415, is located on a projection of a semicircle and outside the outer line of the wall. The Witches Tower had a vaulted upper floor and three upper floors with wooden ceilings. At present it is covered with a reconstructed conical helmet. In the seventeenth century, when the medieval defense devices lost their original function, the tower served as a prison for women who were suspected of witchcraft and hence its current name. From historical sources are also known today nonexistent towers: Oats Tower erected before the Holstein Gate probably in the mid-15th century, and the Powder Tower located in the north-western part of the wall near the Holstein Gate, being a warehouse or powder manufactory.
Słupsk initially had three gates: Mill Gate from the east, Holstein Gate from the north and the New Gate from the west. These gates were built on a rectangular plan with a pointed arches. They were erected in two stages. Initially, in the years 1325-1329, lower floors with passageways and possibly wooden superstructure were built. They coexisted with the earthen shaft ended with a palisade. The replacement of the shaft into a brick-stone wall began probably in 1348. Around 1365-1370 the upper masonry levels of the gates were erected, initially open from the town side. The gates were crowned with ornamental pinnacles.
The Mill Gate has five storeys. Its eastern façade is divided into five elongated, doubled blendes. The western façade is decorated on the sides with double blendes with lancet bows. Side elevations have only arrowlits and blocked windows in the upper storey. New gate closed the town from Sławno, Darłowo and Szczecin. Its upper storey was built in the years 1380-1385. Around 1441 a large pointed opening was walled up from the town side. The tops and gable roof were destroyed during the fire in 1476. Its western elevation in the upper storey is divided into seven vertical panels and finished with a frieze. The east facade is separated by narrow niches ended with pointed arches. In the side elevations there are walled openings, a decorative top frieze and a row of pointed niches.
Over time, as the town fortifications developed, additional gates were created and, in fact, wicket gates. They were the Blacksmith Gate made between 1410-1415 in the eastern part of the wall, consisting of a passageway flanked by a semi-circular tower. The Monks Gate was located in the south-western part of the walls next to the Dominican monastery. It was probably built in the early seventeenth century during the modern modernization of the town’s defense equipment. The Owls Gate, of which the oldest source dates back to 1531, was located near the south-western corner of the wall.
The outer defenses of the town were natural obstacles in the form of the river Słupia with its branches and backwaters and swamps.
The best preserved fragments of the medieval fortifications of Słupsk are the Mill and New Gates and the so-called Witches Tower. There are also fragments of the wall with towers at Jagiełło Street, next to the former Holstein Gate and in the eastern part of the town on the Słupia river. Today, the Museum of Central Pomerania is located in the Mill Gate, and in the New Gate a gallery of contemporary art.
Lukas E, Średniowieczne mury miejskie na Pomorzu Zachodnim. Poznań 1975.
Ptaszyńska D., Miejskie mury obronne w województwie koszalińskim, Koszalin 1974.